To meet the recent demand for higher integration in integrated circuits, pattern formation to a finer feature size is required. Acid-catalyzed chemically amplified resist compositions are most often used in forming resist patterns with a feature size of 0.2 μm or less. High-energy radiation such as UV, deep UV or electron beam (EB) is used as the light source for exposure of these resist compositions. In particular, while EB lithography is utilized as the ultra-fine microfabrication technique, it is also indispensable in processing a photomask blank to form a photomask for use in semiconductor device fabrication.
In general, the EB lithography is by writing an image with EB, without using a mask. In the case of positive resist, those regions of a resist film other than the regions to be retained are successively irradiated with EB having a minute area. The operation of successively scanning all finely divided regions on the process surface takes a long time as compared with full wafer exposure through a photomask. In order to avoid any decline of throughput, the resist film must be highly sensitive. Because of the long image-writing time, there is a likelihood of a difference arising between the initially written portion and the later written portion. Thus the stability with time of exposed regions in vacuum is one of important performance requirements. One of the important applications of chemically amplified resist material resides in processing of mask blanks. Some mask blanks have a surface material that can have an impact on the pattern profile of the overlying resist, such as a film of a chromium compound, typically chromium oxide deposited on a photomask substrate. For high resolution and profile retention after etching, it is one important performance factor to maintain the pattern profile of resist film rectangular independent of the type of substrate.
The control of resist sensitivity and pattern profile as mentioned above has been improved by a proper selection and combination of resist material-constituting components and processing conditions. One problem to be improved is the diffusion of acid that largely affects the resolution of a chemically amplified resist material. In processing of photomasks, it is required that the profile of a resist pattern formed as above do not change with a lapse of time from the end of exposure to post-exposure baking. The major cause of such a change with time is diffusion of an acid generated upon exposure. The problem of acid diffusion has been widely studied not only in the field of photomask processing, but also in the field of general resist materials because it has a significant impact on sensitivity and resolution.
In particular, Patent Document 1 discloses that a sulfonic acid generated upon exposure is incorporated into a resin for use in resist material for inhibiting acid diffusion. This control method is attractive since it relies on a mechanism different from the control method using a base. Various improvements have been made on this method to comply with the demand of forming finer size patterns. Patent Document 2 is a useful example of achieving an improvement in acid strength.